Our long term objective is to understand mechanisms of acoustic information processing in the auditory nervous system. The cat cochlear nucleus provides a model system for this effort because in a general way, its role is to receive incoming auditory nerve discharges, preserve and/or modify the messages, and distribute the resulting output signals to higher brain centers. The structural bases for the signal transformations and the initiation of cellular circuits in this nucleus are hypothesized to be determined largely by the organization of auditory nerve input. We propose to continue the use of intracellular injections of horseradish peroxidase (HRP) into auditory nerve fibers in adult cats. All data gathered to date strongly suggest that only type I spiral ganglion neurons whose peripheral processes innervate inner hair cells are labelled by these techniques. The marking of single fibers allows us to make direct correlations between the fiber's response properties, its morphology and spatial distribution, and the neurons with which it is connected. Since the HRP reaction product is electron dense, these labelled fibers can be examined with an electron microscope and we can ask, for example, whether different physiological groupings of fibers have different synaptic characteristics and/or different connections with cells of the cochlear nucleus. These methods for staining single neurons after characterizing their physiological response properties have enabled us to describe quantitative structure-function relationships at a cellular level. We shall exploit these methods for studying the organization of auditory nerve input to the cochlear nucleus. The compilation of these data should generate new insights into the anatomical basis of stimulus coding in the auditory nerve and cochlear nucleus, and could contribute toward the design of prosthetic devices that successfully bypass nonfunctioning cochleas.